260 



BELL SYSTEM TECHNICAL JOURNAL 



messages in opposite directions travel on the same wire path. The 

 familiar hybrid coil arrangement so common in telephone repeaters 

 and four-wire cable circuits might appear to solve this problem, were 

 there not difficulties peculiar to the radio channels. In the short-wave 

 case large variations in attenuation occur in the radio paths within 

 short intervals of time. These would tend to cause re-transmission of 

 received signals at such amplitudes that severe echoes and even singing 

 around the two ends of the circuit would occur unless means were 

 provided to prevent this. 



To overcome these fundamental transmission difficulties, an auto- 

 matic system of switches operated by the voice currents of the speakers 

 has been developed.^ These devices cut off the radio path in one 



UNITED 

 STATES 



„ BOARD 



RECEIVING 

 AMPLIFIER 



"LI — ' 



RECEIVING 

 DELAY 



TOLL awn 



SWITCH- 



kSiXl 



TSW^ 



hw^ — 



TELEPHONE 

 OPERATOR 



HYBRID COIL 



TESQ- 



RADIO 

 RECEIVER 



ETCOr 



J- 



FROM 

 ENGLAND 



a 



RECEIVING 

 DETECTOR 



w. 



TRANS- 

 MITTING 

 DETECTOR 



TRANS- 

 MITTING 

 DELAY 



LAWRENCEVILLE 



CO 



2 



RADIO 



-► TRANS- 

 MITTER 



J- 



TO 

 ENGLAND 



Fig. 1^ — Circuit diagram illustrating operation of voice-operated switching device. 



direction while speech is traveling in the reverse direction and also keep 

 one direction blocked when no speech is being transmitted. The 

 operation is so rapid that it is unnoticed by the telephone users. Since 

 this system prevents the existence of singing and echo paths, it permits 

 the amplification to be varied at several points almost without regard 

 to changes in other parts of the system, and it is possible by manual 

 adjustment to maintain the volumes passing into the radio link at 

 relatively constant values, irrespective of the lengths of the connected 

 wire circuits and the talking habits of the subscribers. 



Fig. 1 gives a schematic diagram of the United States end of one of 

 the short-wave circuits showing the essential features of a voice- 

 operated device which has been used. This kind of apparatus is 



^ For detailed description of this system see "The New York-London Telephone 

 Circuit" by S. B. Wright and H. C. Silent, Bell System Tech. JL, Vol. VI, October, 

 1927, pp. 736-749. 



